Methods by means of which direct positive images are obtained using internal latent image type silver halide emulsions which have not been pre-fogged by carrying out imagewise exposure followed by surface development after carrying out a fogging process or while carrying out a fogging process are well known.
Herein, the term "internal latent image type silver halide emulsion" signifies a silver halide of the type in which the photosensitive nuclei are principally within the silver halide grains, and in which the latent image formed by exposure to light is formed principally within the grains.
Various techniques are known in this field of technology. For example, the principal techniques have been disclosed, for example, in the specifications of U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276, 3,796,577, 1,151,363, 1,150,553 and 1,011,062.
It is possible to provide comparatively fast direct positive photographic materials using these known techniques.
Details of the mechanism by which the above mentioned direct positive images are formed have been described, for example, in T. H. James The Theory of the Photographic Process, volume 4, chapter 7, pages 182-193, and in U.S. Pat. No. 3,761,276.
The inventors have already invented a method in which high contrast direct positive photosensitive materials which have a low Dmin value obtained by including thiosulfonic acid in the emulsion, and a patent application has been made in this connection (Japanese Patent Application No. 63-83677).
Moreover, various additives are added to the photographic materials in general in order to improve their photographic characteristics, and the use of mixtures of thiosulfonic/sulfinic acids in black-and white negative photosensitive materials has been disclosed in U.S. Pat. No. 2,394,198, where mainly sulfinic acid with just a small amount of thiosulfonic acid is added.
In order to be practical, direct positive photographic materials must have a high Dmax, a low Dmin and a high contrast. However, when additives are added to direct positive photographic materials in particular to reduce Dmin they also tend to reduce the value of Dmax.
High contrast direct positive photographic materials which have a low Dmin value can be obtained with the method of the aforementioned Japanese Patent Application No. 63-83677, but these materials have disadvantages, namely (1) that the fresh speed is low, and (2) that the change in speed (sensitization) on aging during storage is also considerable.